1. Field of the Invention
The present invention relates to a boost circuit and a voltage generating apparatus and an electronic apparatus in which the boost circuit can be used.
2. Description of the Related Art
Battery-driven portable equipment such as a portable telephone and a personal data assistant (PDA) use light-emitting diodes (LED) for a variety of purposes. For example, LEDs are used to provide backlight for a liquid crystal display (LCD) or an electronic flash light for a charge-coupled device (CCD) camera. LEDs emitting different colors are operated to blink for illumination. In order to drive an LED, the battery voltage of about 3.6V of, for example, a lithium ion battery should be boosted to about 4.5V as a drive voltage. When the battery voltage drops due to battery consumption, or when a load current flowing in the LEDs is increased, causing an increased voltage drop, the battery voltage should be boosted by a higher step-up ratio.
Thus, in the power supply apparatus for driving a target load such as LEDs, the battery voltage should be boosted by an appropriate step-up ratio in accordance with the operating condition so as to generate a drive voltage. For example, the patent document No. 1 discloses a drive voltage supplying apparatus in which a boost circuit provided with a plurality of boost capacitors is also provided with a selection switch that selects appropriate boost capacitors depending on the step-up ratio and an external selection terminal connected to the selection switch for selection of a step-up ratio.
[Patent Document No. 1]
Japanese Published Patent Application No. 6-78527
The apparatus according to the patent document No. 1 is certainly capable of boosting the power supply voltage by a desired step-up ratio. However, an excessive current may temporarily flow due to initial charge built up in the boost capacitors. Such a current is referred to as an inrush current. An inrush current may adversely affect the reliability of a power supply apparatus itself or equipment including the power supply apparatus. It may also induce improper operation of the equipment. Therefore, it is preferable that an inrush current be suppressed. Further, for control of power consumption of a battery used as a power supply for input to a power supply apparatus, it is preferable to control a consumption current in the circuit.